This invention relates generally to operation of a catalytic tubular reactor to reform a mixture of hydrocarbon and steam to produce hydrogen for consumption in a fuel cell; and more particularly concerns maintenance of the temperature of a reactor burner surface temperature within defined limits, by use of a control responsive to fuel cell load changes to control hydrocarbon feed to the fuel cell.
Proton exchange membrane (PEM) fuel cells have emerged as a viable option for the production of disbursed electrical power, typically in the range of 1-50 kW, for use in residential and small commercial applications. PEM fuel cells generate electricity by the electrochemical reaction between hydrogen and oxygen. While oxygen is readily available from ambient air, hydrogen must be produced from commercially available fuels, such as natural gas or propane, using methods such as steam reforming. Steam reforming is a process that involves a high temperature catalytic reaction between a hydrocarbon and steam to form a hydrogen-rich product gas that contains significant quantities of carbon monoxide.
Because PEM fuel cells have a low tolerance to carbon monoxide, typically less than 10 ppm, additional processing steps are required to prepare a hydrogen-rich gas stream that is suitable for use in a PEM fuel cell. These additional steps typically include application of one or more stages of water-gas shift reaction that ultimately reduce the carbon monoxide concentration to less than 10,000 ppm, and a selective oxidation reaction step that further reduces the carbon monoxide concentration to less than 10 ppm. The physical embodiment of the process equipment that achieves the combination of reaction steps needed to convert the hydrocarbon feed to a useful hydrogen product is commonly referred to as a fuel processor.
As an illustration, Table 1 summarizes the reaction steps of a fuel processor designed to produce a hydrogen-rich gas stream suitable for use in a PEM fuel cell.
Table 1. PEM fuel processor reactions steps
Methods to generate hydrogen from hydrocarbon fuels for industrial purposes using the combination of steam reforming and water-gas shift reaction steps are generally known in the prior art. However, industrial hydrogen generators generally do not require a capability for rapid changes in the hydrogen generation capacity. It is desirable that the PEM fuel cell deliver electricity upon demand and thus the fuel processor must be capable of delivering variable quantities of hydrogen to the PEM fuel cell stack in accordance with the electric load requirements, which can change rapidly.
The present invention concerns provision of a compact fuel processor comprising a catalytic tubular reactor that is heated using an infrared radiant burner to provide the endothermic heat of reaction needed to reform a mixture of hydrocarbon and steam for the production of a hydrogen-rich gas stream. The hydrogen-rich gas stream is further purified using a sequence of catalytic steps and is fed to a fuel cell whereupon a portion of the hydrogen contained in the gas stream is consumed for the production of electricity by electrochemical reaction with oxygen. An unused portion of the purified hydrogen-rich gas stream exits the fuel cell stack and is combusted in the infrared radiant burner. A fuel cell control rapidly responds to a variable fuel cell electric demand by adjusting the feed of hydrocarbon to the catalytic tubular reactor to maintain the surface temperature of the infrared radiant burner within defined limits.